In the industrial facilities such as nuclear power plants, many kinds and sizes of bolts are used. But bolt degradation has become a major issue in the nuclear industry since the 1980's due to failure during operation. If small cracks in stud bolts are not detected early enough, they grow rapidly and cause catastrophic disasters. Their detection, despite its importance, is known to be a very difficult problem due to the complicated structures of the stud bolts.
Generally, ultrasonic, magnetic particle and eddy current testing procedures are carried out for bolt inspection. Among these, ultrasonic inspection is the only one which is expected to detect cracks in the thread region, under the condition that the studs and bolts are not removed. However, by conventional ultrasonic testing methods, it is difficult to detect flaws such as stress-corrosion cracks or corrosion wastage in the threads. In many cases, a small flaws signal can hardly be distinguished from the complicated signals reflected from threads. When the flaw is quite small, the signal amplitude reflected from it is nearly equal in size to the noise level.
Therefore, there exits a need for a method for detection of cracks in a stud bolt that distinguishes the small signal from the flaws in the stud bolt from the complicated signals reflected from threads of the stud bolt.